Asymmetric Nanotube Containing Membranes

ABSTRACT

This invention relates to heterogenous pore polymer nanotube membranes useful in filtration, such as reverse osmosis desalination, nanofiltration, ultrafiltration and gas separation.

BACKGROUND OF THE INVENTION

1 . Field of the Invention

This invention relates to heterogenous pore polymer nanotube membranesuseful in filtration, such as reverse osmosis desalination,nanofiltration, ultrafiltration and gas separation,

2. Description of the Prior Art

Polymer membranes with pores in the polymer surface are often used inreverse osmosis purification of fluids, such as water. These membranespermit transport of water through a solution-diffusion mechanism. Adisadvantage is that water must dissolve into the polymer materialcomprising the membrane and diffuse through. The result is a very lowwater flux compared to membrane materials that contain open channelsthrough the membrane.

To increase flux in conventional membrane materials, the constituentpolmer structure can be made looser by judicious choice ofpolymerization parameters. However, the rejection performance of themembrane is reduced as a result. In some cases, materials have beenadded to the polymer to adjust permeability properties. For example,U.S. Pat. No. 4,277,344 (J. E. Cadotte, Film Tec Corporation) describesinterfacial synthesis of a reverse osmosis membrane with embeddedparticles. US patent application no. 2006/007037 (The Regents of theUniversity of California) is a variant of the Cadotte patent, describingmethods of membrane fabrication. These membranes remain unsatisfactorysince increasing permeability will often reduce selectivity offiltration.

A different type of membrane involves directing water flux throughcarbon nanotubes attached to a silicon chip. US patent publication no.2006/033180 (The Regents of the University of California) describes amethod of fabricating a material using micromachining or microelectromechanical systems techniques. Heft et al. (Science 312, 1034(2000) describes water transport through sub-2 nm inner diameter carbonnanotube pores as being higher than predictions of continuumhydrodynamics models. The method described is a micro-electro-mechanicalsystems compatible fabrication process for fabrication of carbonnanotube pore membranes using catalytic chemical vapor deposition growthof a dense, vertically-aligned array of double walled carbon nanotubeson the surface of a silicon chip. Gaps between the nanotubes are thenfilled in a separation step by a process such as vapor deposition.However, this method presents problems with respect to scalability, dueto the use of the chemical vapor deposition, and cost, due to the use ofsilicon as a substrate material.

There remains a need for a sealable filtration membrane that providesadequate flux and selectivity for commercial use in desalination,nanofiltration, and ultrafiltration.

SUMMARY OF THE INVENTION

One aspect of the invention is a is a membrane of selectivepermeability, which membrane comprises a porous polymer and terminatingon one side in a skin, with nanotubes embedded in said membrane andprotruding through said skin and said skin forming a substantiallyimpermeable barrier around said nanotubes, said membrane having poresincreasing in diameter with increasing distance from said skin.

A second aspect of the invention is a membrane of selectivepermeability, which membrane comprises a porous polymer mid terminatingon one side in a skin and terminating on the opposite side from saidskin in a porous non-woven substrate, with nanotubes embedded in saidmembrane and protruding through said skin and said skin forming asubstantially impermeable barrier around said nanotubes, said membranehaving pores increasing in diameter with increasing distance from saidskin.

A third aspect of the invention is a process for the fabrication of amembrane of selective permeability, said process comprises: (a) coatinga substrate with a film of polymer solution comprising a polymerdissolved in a solvent; (b) forming over said film of polymer solution ananotube dispersion layer comprising nanotubes dispersed in a liquidcarrier that is partially miscible with, but forms a separate liquidphase from, said polymer solution; (c) evaporating said liquid carrierfrom said nanotube dispersion layer to leave a residual nanotubedispersion layer over said substrate, said residual nanotube dispersionlayer comprising said nanotubes dispersed in said polymer solution andprotruding from said nanotube dispersion layer; (d) contacting saidresidual nanotube dispersion layer with a liquid that is at leastpartially miscible with said solvent but in which said polymer issubstantially insoluble, to cause precipitation of said polymer to forma porous membrane terminating on one side in a skin, with balder aroundsaid nanotubes, said membrane having pores increasing in diameter withincreasing distance from nanotube said skin; and (e) optionallyseparating said porous membrane from said substrate.

DETAILED DESCRIPTION OF TILE INVENTION Definitions

Unless otherwise stated, the following terms used in the specificationand claims are defined for the purposes of this Application and have thefollowing meanings.

“Nanotubes” are cylindrical tubular structures that are of micrometerscale. Nanotubes of a variety of materials have been studied, notablycarbon nanotubes, boron nanotubes, and nanotubes of boron nitride. Thosethat have been most extensively studied are carbon nanotubes, whosefeatures and methods of fabrication are illustrative of nanotubes ingeneral.

“Carbon nanotubes” are polymers of pure carbon, and exist as single-walland multi-wall structures. Examples of publications describing carbonnanotubes and their methods of fabrication are Dresselhaus, M. S., etal., Science of Fullerenes and Carbon Nanotubes, Academic Press, SanDiego (1996). Ajayan, P. M., et al., “Nanometre-Size Tubes of Carbon,”Rep. Frog. Phys. 60 (1997): 1025-1062, and Peigney, A., et al., “Carbonnanotubes in novel ceramic matrix nanocomposites,” Ceram. Inter. 26(2000) 677-683. A single-wall carbon nanotube is a single grapheme sheetrolled into a seamless cylinder with either open or closed ends. Whenclosed, the ends are capped either by half fullerenes or by more complexstructures such as pentagonal lattices. The average diameter of asingle-wall carbon nanotube typically ranges of 0.6 nm to 100 nm, and inmany cases 1.5 nm to 10 nm. The aspect ratio, i.e., length to diameter,typically runs from about 25 to about 1,000,000, and most often fromabout 100 to about 1,000. A of 1 nm diameter may thus have a length offrom about 100 to about 1,000 nm. Nanotubes frequently exist as “ropes,”which are bundles of 3 to 500 single-wall nanotubes held together alongtheir lengths by van der Waals forces. Individual nanotubes often branchoff from a rope to join nanotubes of other ropes. Multi-walled carbonnanotubes are two or more concentric cylinders of graphene sheets ofsuccessively larger diameter, forming a layered composite tube bondedtogether by van der Waals forces, with a distance of approximately 0.34nm between layers.

Carbon nanotubes can be prepared by arc discharge between carbonelectrodes in an inert gas atmosphere. This process results in a mixtureof single-wall and multi-wall nanotubes, although the formation ofsingle-wall nanotubes can be favored by the use of transition metalcatalysts such as iron or cobalt. Single-wall nanotubes can also beprepared by laser ablation, as disclosed by Thess, A., et al.,“Crystalline Ropes of Metallic Carbon Nanotubes,” Science 273 (1996):483-487, and by Witanachi, S., et al., “Role of Temporal Delay inDual-Laser Ablated Plumes,” J. Vac. Sci. Technol. A 3 (1995): 1171-1174.A further method of producing single-wall nanotubes is the high-pressurecarbon monoxide conversion (“HiPCO”) process disclosed by Nikolaev, P.,et al., “Gas-phase catalytic growth of single-walled carbon nanotubesfrom carbon monoxide,” Chem. Phys. Lett. 313, 91-97 (1999), and byBronikowski, M. J., et al., “Gas-phase production of carbonsingle-walled nanotubes from carbon monoxide via the HiPCO process: Aparametric study,” J. Vac. Sci. Technol. 19, 1800-1805 (2001).

Certain procedures for the synthesis of nanotubes will produce nanotubeswith open ends while others will produce closed-end nanotubes. If thenanotubes are synthesized in closed-end form, the closed ends can heopened by a variety of methods known in the art. An example of ananotube synthesis procedure that produces open-ended nanotubes is thatdescribed by Hua, D. H. (Kansas State University Research Foundation),International Patent Application Publication No. WO 2008/048227 A2,publication date Apr. 24, 2008. Closed ends can be opened by mechanicalmeans such as cutting, by chemical means or by thermal means. An exampleof a cutting method is milling. Chemical means include the use of carbonnanotube degrading agents, an example of which is a mixture of a nitricacid and sulfuric acid in aqueous solution at concentrations of up to70% and 96%, respectively. Another chemical means is reactive ionetching. Thermal means include exposure to elevated temperature in anoxidizing atmosphere. The oxidizing atmosphere can be achieved by anoxygen concentration ranging from 20% to 100% by volume, and thetemperature can range from 200° C. to 450° C.

The lengths of the nanotubes can vary widely and are not critical to theinvention. The lengths are expressed herein as average lengths, usingnumerical or arithmetic averages. In preferred embodiments, the averagelength is from about 100 nm to about 2000 nm, most preferably from about200 nm to about 1000 nm, whether single-wall, multi-wall, or acombination of single-wall and multi-wall. The outer and inner diametersof the nanotubes can likewise vary. In the most common embodiments, theouter diameters can range from about 0.6 nm to about 200 nm, whilenarrower ranges are often preferred for particular applications. Theinner diameters in the most common embodiments can likewise range fromabout 0.4 nm to about 200 nm, although the optimal diameters forparticular applications may be within narrower ranges. For reverseosmosis, and notably for water desalination, a preferred inner diameterrange is about 0.4 nm to about 5 nm, and a most preferred range is frontabout 0.4 nm to about 1.2 nm. For nanofiltration membranes, a preferredsize range is from about 1 nm to about 10 nm. For ultrafiltrationmembranes, a preferred size range is from about 5 nm to about 200 nm.

“Polymers” useful in preparing the asymmetric membranes of the inventioninclude, but are not limited to, aromatic polyamides, aliphaticpolyamides, cellulose acetates, cellulose nitrate, cellulosicderivatives, ethyl cellulose, polyesters, polycarbonates,copolycarbonate esters, polyethers, polyetherketones, polyetherimides,polyethersulfones polyetheresters, polysulfones, polyvinylidenefluoride, polybenzimidazoles, polybenzoxazoles, polyacrylonitrile,polyazoaroaromatics, poly(2,6-dimethylphenylene oxide), polyphenyleneoxides, polyureas, polyurethanes, polyhydrazides, polyazomethines,polyacetals, styrene-acrylonitrile copolymers, brominated poly(xyleneoxide), sulfonated poly(xylylene oxide), polyquinoxaline, polyamideimides, polyamide esters, polysiloxanes, polyacetylenes such aspoly(trimethylsilylpropyne), polyphosphazenes, polyolefines such aspolyethylene, polypropylene and poly(4-methylpentene), polyphenylenes,polyimides, polyesters and so called ladder polymers, polyacrlonitrile,polyphthalamides, polysulfonamides, polyamide-imides, phenylene diaminessuch as ortho-phenylenediamine and meta-phenylenediamine, Matrimid®,Lenzing P84, polyamide hydrazide, Nylon 6, poly(ethylene-co-vinylalcohol), polytetrafluoroethylene, and the like and any blends,copolymers, and substituted polymers thereof. Polymers preferred forpreparing the asymmetric membranes of the invention are polysulfones,e.g., poly(1,4-phenylene ether-ether-sulfone),poly(1-hexadecene-sulfone), poly(1-tetradecene-sulfone),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly[1-[4-(3-carboxy-4-hydroxyphenylazo)beazonesulfonamido]-1,2-ethanediyl,polyphenylsulfone and polysulfone.

“Selective permeability” as used herein means molecules of a specifiedsize can freely pass through the membrane while most, in not allmolecules of larger sizes cannot pass through the membrane.

“Liquid carrier”is a liquid composition that is at least partiallymiscible with the solvent containing the polymer comprising the membraneand wherein most of the nanotubes are dispersed in solution such thatwhen the solution of nanotubes is formed over the surface of the film ofthe polymer solution the nanotubes become evenly embedded into theunderlying polymer solution. Suitable compositions may contain ananotube dispersing agent.

“Substantially impermeable barrier” as used herein means a barrier that,except for the nanotubes embedded in the barrier, is either completelyimpervious or partially impervious to the extent that the barrier ispervious to molecules that are smaller than or about equal to in size ofthose molecules that can freely pass through the embedded nanotubes.

“Substrate” as used herein for the casting surface on which the film ofpolymer solution is coated can be comprised of any non-reactive materialthat the liquid polymer solution will adhere to during the casting andimmersion steps. Suitable substrates may be non-porous in which ease thematerial must be such that the membrane formed by the process can easilyhe separated, e.g., glass, and the like. Suitable substrates can beporous in which case the substrate would not generally separated fromthe membrane formed by the process and would be an integral component ofthe membrane. A suitable porous substrate includes non-woven fabric.

Preferred Embodiments

While the broadest definition of the invention is set forth in theSummary of the Invention, certain aspects of the invention arepreferred. For example, a preferred embodiment is a membrane ofinvention wherein the polymer used for making the heterogenous porepolymer structure is selected from the group consisting ofpoly(1,4-phenylene ether-ether-sulfone), poly(1-hex adecene-sulfone),poly(1-tetradecene-sulfone),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylensulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl,polyphenylsulfone and polysulfone. Preferably the polymer ispolysulfone.

Preferred membranes are those in which the nanotubes are carbonnanotubes. Preferred membranes are those wherein the embedded nanotubesare single-walled nanotubes. Preferred membranes are those wherein theaverage length of the nanotubes is from about 300 nm to about 2000 nm,more preferably from about 500 nm to about 1000 nm. Preferred membranesare those wherein the nanotubes have inner diameters from about 0.4 nmto about 20 nm, more preferably from about 0.8 nm to about 10 nm, morepreferably from about 0.8 nm to about 1.4 nm and most preferably fromabout 0.8 nm to about 0.9 nm.

A preferred process for making the asymmetric membranes of the inventionis wherein the dispersion solvent contains a nanotube dispersing agent.Preferably the dispersing agent is polystyrene-poly-3-hexylpolythiophenecopolymer. Preferably the polymer used in the process to make theheterogenous porous polymer structure is a polysulfone-like polymerselected from the group consisting of poly(1,4-phenyleneether-ether-sulfone), poly(1-hexadecene-sulfone),poly(1-tetradecene-sulfone),poly(oxy-1,4-phenylenesulfonly-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly[1-4-(3-carboxy-4-hydroxyphenylazo)-benzenesulfonamido]-1,2-ethanediyl,polyphenylsulfone and polysulfone. Preferably polysulfone is the polymerused in the process.

Preparation of Membranes

The membranes of the invention are asymmetric membranes and the stepscomprising the process for preparing the membranes of the invention, ingeneral, can be performed by methods known in the art. The membranes aremade by phase inversion using immersion precipitation techniques,whereby a east polymer in liquid state is transformed into a solid stateby immersion in a non-solvent. The immersion precipitation produces amembrane having a thin surface layer overlaying a porous sublayer. Avariety of process variables will impact the physical characteristics ofthe membrane as a whole. Polymer type, polymer concentration, polymersolvent, nanotube solvent, precipitation liquid, dry times andtemperatures differentially affect the process.

Accordingly, membranes of the invention are prepared by (a) coating asubstrate with a film of polymer solution comprising a polymer dissolvedin a solvent; (b) forming over said film of polymer solution a nanotubedispersion layer comprising nanotubes dispersed in a liquid carrier thatis partially miscible with, but forms a separate liquid phase from saidpolymer solution; (c) evaporating said liquid carrier from said nanotubedispersion layer to leave a residual nanotube dispersion layer over saidsubstrate, said residual nanotube dispersion layer comprising saidnanotubes dispersed in said polymer solution and protruding from saidnanotube dispersion layer: (d) contacting said residual nanotubedispersion layer with a liquid that is at least partially miscible withsaid solvent but in which said polymer is substantially insoluble, tocause precipitation of said polymer to form a porous membraneterminating on one side in a skin, with barrier around said nanotubes,said membrane having pores increasing in diameter with increasingdistance from said skin and (e) optionally separating said porousmembrane from said substrate.

Coating of the liquid polymer solution is carried out by spreading athin layer of the polymer solution onto a substrate. The coating iscarried out at −10 to 90° C., preferably at about ambient temperature.The polymer solvent type will be dictated by the polymer type and theprecipitation liquid. Suitable solvents, in general, include polaraprotic solvents, e.g. N,N-dimethylacetamide, N,N-dimethylformamide,dimethyl sulfoxide, N-methyl-2-pyrrolidinone, and the like. Polymerconcentrations will vary with the polymer type. Typically the polymerconcentration will be from about 10 to about 30% by weight andpreferably about 20 by weight. The polymer is spread onto the substrateto a thickness of 0.001 to 1 mm and preferably to about 0.25 mm. Thepolymer layer is allowed to set on the substrate about 2 seconds beforethe nanotube solution is applied.

Deposition of the nanotube solution is carried out by spreading a thinlayer of the solution onto the surface of the liquid polymer layer. Thedeposition is carried out at 10° C. to 70° C., preferably at aboutambient temperature. Suitable solvents, in general, include solvents,e.g., chloroform, toluene, benzene, halobenzenes, alkyl benzenes,tetrahydrofurna, and the like. Preferably the dispersion solvent is atleast partially miscible with the solvent containing the polymercomprising the membrane and wherein most of the nanotubes are dispersedin solution as isolated nanotubes. Preferred dispersion solvents containa nanotube dispersing agent. A preferred dispersing agent ispolystyrene-poly-3-hexylpolythiophene copolymer.

Evaporation of the solubilizing solvent can be carried out by forcing aconvective airflow over the liquid polymer layer or allowing the solventto evaporate freely into the air. The evaporation can be effected atambient temperature to 50° C. and requires 5 seconds to 15 minutes tocomplete. A preferred method for drying the multilayer polymer depositis in an oven at 70° C. for about 10 minutes

The precipitation step is carried out at ambient temperature to −10° C.and requires 5 minutes to 15 minutes to complete. Suitable precipitationliquids are those that are a non-solvent for the polymer. Suitablenon-solvents are polar solvents, e.g. water, alcohols, glycols, and thelike or suitable mixtures thereof. The precipitation step is carried outin a manlier that a thin skin layer at the surface of the heterogeneouspore structure forming a substantially impermeable barrier around thenanotubes. The thin skin layer will be thick enough to be substantiallyimpermeable and thin enough such that some or all of the nanotubes willhave each of their open ends protruding from opposite sides of andproviding selective egress through the barrier. Preferably, the thinskin surface layer has a minimum thickness of one-third, and morepreferably one-fifth, of the average nanotube length.

Separation of the heterogeneous pore structure from the substrate can becarried out by any means that separates the membrane from the substratewhile maintaining an intact membrane. Typically the membrane is gentlypeeled away from the substrate.

EXAMPLES

The following examples arc offered for purposes of illustration and arenot intended to limit the scope of the invention.

Example 1 Preparation of Asymmetric Cellulose Acetate/Nanotube Membrane

A solution of 18% by weight cellulose diacetate (acetylation 39.8%) in a2.5:1 volume ratio of acetone to formamide was spread with a castingknife onto a surface of a glass plate to a thickness of about 0.25 mm.The liquid cellulose acetate layer was allowed to set on the glasssurface for 2 seconds and then a 0.1% by weight solution ofmulti-walled, open-ended nanotubes dispersed using 0.1% by weightpolystyrene-poly 3 hexylthiophene in chloroform was spread with acasting knife onto the liquid cellulose acetate layer to a thickness ofabout 0.01 mm. The multilayer wet film was exposed to dry air for 15seconds to allow the chloroform to evaporate and the carbon nanotubes tobecome embedded in the incipient skin layer of the membrane. The wetfilm was then immersed in water to drive the formation of theasymmetric. membrane. The coagulated membrane separated from the glasssurface and was removed to yield a cellulose acetate/nanotube membranemeasuring from 0.06 to 0.10 mm in thickness. Scanning electronmicroscopy revealed that the membrane had an asymmetric pore structurewith an approximately 200 nm skin layer having no discernible poressupported by a heterogeneous sponge-like layer underneath. The carbonnanotubes were concentrated in the topmost layer wherein some of thenanotubes embedded within the skin had one open end protruding above theskin layer and the other open end traversing through the thin skin outerlayer into the porous region beneath the thin skin outer layer.

Example 2

Preparation of Asymmetric Polysulfone/Nanotube Membrane

A solution of 15% by weight polysulfone (Udel-3500) inN-Methyl-2-pyrrolidone was spread with a casting knife onto a surface ofa glass plate to a thickness of about 0.25 mm. The liquid polysulfonelayer was allowed to set on the glass surface for 2 seconds and then a0.1% by weight solution of multi-walled, open-ended nanotubes dispersedusing 0.1% by weight polystyrene-poly 3 hexylthiophene in toluene wasspread with a casting knife onto the liquid cellulose acetate layer to athickness of about 0.01 mm. The multilayer wet film was annealed in anoven at 70° C. for 10 minutes to allow the toluene to evaporate and thecarbon nanotubes to become embedded in the incipient skin layer of themembrane. The wet film was then immersed in 10° C. water to drive theformation of the asymmetric membrane. The coagulated membrane separatedfrom the glass surface and was removed to yield a polysulfone/nanotubemembrane measuring from 0.06 to 0.10 mm in thickness. Scanning electronmicroscopy revealed that the membrane bad an asymmetric pore structurewith an approximately 250 nm skin layer supported by a heterogeneoussponge-like layer underneath. The carbon nanotubes were concentrated inthe topmost layer wherein some of the nanotubes embedded within the skinhad one open end protruding above the skin layer and the other open endtraversing through the thin skin outer layer into the porous regionbeneath the thin skin outer layer. Permeabilities of thepolysulfone/carbon nanotube membranes were 3.8 times higher than thepermeabilities of the non-carbon nanotube polysulfone controls whilerejection of a 3.5 nm marker was reduced, indicating that sore liquidtransport was occurring through the carbon nanotubes while some liquidtransport was still occurring through pores in the polymer membrane.

TABLE 1 Comparison of Polysulfone/Carbon Nanotube Performance vs.Polysulfone controls Permeability Rejection Membrane Type (m³/m²-s-Pa)(% Peg4000) Polysulfone/CNT Membrane 373 ± 96 × 10⁻¹² 39 Polysulfonecontrol  96 ± 22 × 10⁻¹² 80

I the claims appended hereto, the term “a” or “an” is intended to mean“one or more.” The term “comprise” and variations thereof such as“comprises” and “comprising,” when preceding the recitation of a step oran element are intended to mean that the addition of steps or elementsis optional and not excluded. All patents, patent applications, andother published reference materials cited in this specification arehereby incorporated herein by reference in their entirety. Anydiscrepancy between any reference material cited herein or any prior artin general and an explicit teaching of this specification is intended tobe resolved in favor of the teaching in this specification. Thisincludes any discrepancy between an art-understood definition of a wordor phrase and a definition explicitly provided in this specification ofthe same word or phrase.

1. A filter, comprising: a porous polymer terminating on one side in apolymer skin, wherein the porous polymer and the polymer skin comprisethe same polymer; and open-ended nanotubes embedded in the polymer skin,at least a portion of the nanotubes each having two open ends protrudingthrough opposite surfaces respectively of the polymer skin to providefluid communication through each of the nanotubes; wherein the polymerskin forms a substantially impermeable barrier around the nanotubes. 2.The filter of claim 1, wherein the polymer is selected from the groupconsisting of poly(1,4-phenylene ether-ether-sulfone),poly(1-hexadecene-sulfone), poly(1-tetradecene-sulfone),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl,polyphenylsulfone and polysulfone.
 3. The filter of claim 1, wherein theporous polymer has pores increasing in diameter with increasing distancefrom the polymer skin.
 4. The filter of claim 1, wherein the nanotubesare single walled carbon nanotubes.
 5. The filter of claim 1, whereinthe polymer skin has a minimum thickness of one-fifth of the averagenanotube length.
 6. The filter of claim 5, wherein the average length ofthe nanotubes is from 300 nm to 2000 nm.
 7. The filter of claim 1,wherein the nanotubes have inner diameters from 0.4 nm to 20 nm.
 8. Thefilter of claim 1, wherein the filter comprises a reverse osmosisdesalination filter, a nanofiltration filter or an ultrafiltrationfilter.
 9. The filter of claim 1, wherein the nanotubes have asubstantially random orientation relative to the polymer skin.
 10. Thefilter of claim 1, wherein the nanotubes protrude into the porouspolymer through one surface of the polymer skin.
 11. A filter,comprising: an asymmetric porous polymer terminating on one side in askin; and open-ended nanotubes embedded in the skin and protrudingthrough opposite surfaces of the skin to provide fluid communicationthrough each of the nanotubes; wherein the skin forms a substantiallyimpermeable barrier around the nanotubes, and the polymer has poresincreasing in diameter with increasing distance from the skin.
 12. Thefilter of claim 11, wherein the polymer is selected from the groupconsisting of poly(1,4-phenylene ether-ether-sulfone),poly(1-hexadecene-sulfone), poly(1-tetradecene-sulfone),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl,polyphenylsulfone and polysulfone.
 13. The filter of claim 11, whereinthe porous polymer and the skin comprise the same polymer.
 14. Thefilter of claim 11, wherein the nanotubes are single walled carbonnanotubes.
 15. The filter of claim 11, wherein the skin has a minimumthickness of one-fifth of the average nanotube length.
 16. The filter ofclaim 15, wherein the average length of the nanotubes is from 300 nm to2000 nm.
 17. The filter of claim 11, wherein the nanotubes have innerdiameters from 0.4 nm to 20 nm.
 18. The filter of claim 11, wherein thefilter comprises a reverse osmosis desalination filter, a nanofiltrationfilter or an ultrafiltration filter.
 19. The filter of claim 11, whereinthe nanotubes have a substantially random orientation relative to theskin.
 20. The filter of claim 11, wherein the nanotubes protrude intothe porous polymer through one surface of the skin.